1. Field of the Invention
The present invention relates to the field of ultrasonic scanners and in particular, to transducers used in medical diagnostic ultrasonic scanners.
2. Prior Art
Ultrasonic scanning instruments are used in medical diagnostics to view regions or particular organs within the body without the necessity of surgical incision to expose the area of interest. In its most fundamental operation, the ultrasonic scanning instrument is placed in contact with the surface of the body to be examined. The scanning instrument then emits a series of pulses, at an ultrasonic frequency, into the body being examined. During the time between the emission of the pulses, the instrument searches for and detects echoes of the emitted pulses which have been reflected by the various internal objects of interest. It is these echoes and their relationship to the emitted pulses which generates a representation or a "view" of the internal region or organs of interest.
A problem which has continued to plague the design and use of ultrasound transducers in medical diagnostic imaging relates to the compromise which must be made between the lateral resolution that can be achieved by such transducers and the range over which good lateral resolution can be maintained.
One prior art attempt to improve the lateral resolution of such transducers was to focus such transducers generally by curving the transducer crystal. For a circular transducer of diameter D operating at a wavelength .lambda., the transducer can be focused out to a focal distance F such that F&lt;D.sup.2 /4.lambda.. The lateral (Rayleigh) resolution achieved at the focus will then be .rho..sub.F .congruent.1.22(F.lambda./D). The depth of focus of a transducer, i.e., the length of the region over which the resolution .rho. remains within some set factor of .rho..sub.F is .DELTA.=k(F/D.sup.2).lambda. (k=7 for the so-called `6 db down` criterion).
It can be seen from the equations for .rho..sub.F and .DELTA. that, for a given wavelength .lambda., decreasing F/D (e.g., by increasing the size of the transducer) will decrease .rho..sub.F which is desirable, but will also decrease the depth of focus .DELTA., which is undesirable. Similarly, increased depth of focus .DELTA. is obtained at the expense of an increase in .rho..sub.F.
Another prior art attempt which addressed this compromise between resolution and the depth of acceptable resolution was the replacement of a circular transducer of diameter D with a very narrow (typically one wavelength wide) annulus transducer of equal radius D. The resolution of such an annulus at any distance x beyond the first few diameters from its face is equal to K(x.lambda./D); that is, such an annulus is essentially focused at all depths except near the transducer. The annulus suffers from several serious drawbacks, however, among them being that it has a poor beam pattern close to the transducer, large side lobes in its beam pattern and poor sensitivity because of its reduced area.
The limitations of an annular transducer can be minimized somewhat by use of a coaxial transducer wherein the central inner disk of the transducer is used as the transmitter and an annulus used as the receiver. See, for example the article by Reginald C. Eggleton entitled "Single Transducer Ultrasound Imaging", appearing in Medical Physics, Volume 3, No. 5, p. 303 (1976). The overall pattern of this transducer is the product of the transmit and receive patterns; thus, the inner disk will have the usual limited focal depth pattern, but will have minimal sidelobes, while the annular array will exhibit good resolution throughout the depth. This combination thus results in reduction (but not elimination) of the sidelobes at the expense of some loss of focal depth (compared to the annulus alone). However, it still does not resolve the problem of poor resolution within the first few diameters from the transducer face, and more seriously, the receiver sensitivity remains quite poor, because of the greatly reduced receiver area (typically a tenth to a twentieth of the full aperture).
Another approach which has been implemented to achieve high resolution over an acceptable range is by the use of phased annular arrays. In such arrays the single transducer crystal is replaced by a set of independently energized transducer elements in the shape of annuli. Typically ten elements are utilized, each connected to suitable transmit/receive electronics by means of variable delay lines. By this arrangement, the transmitted beam focus can be set at a selected value by appropriate delay in the transmit mode, while in the receive mode, the transducer can be dynamically focused by quasi-continuous variation of the interelement delays so as to `track` the transmitted pulse as it travels into the object to be examined.
However, phased annular arrays also suffer from significant limitations. These limitationss include: cost and complexity; poor signal to noise characteristics; limited dynamic range; and relatively large sidelobes. Thus, the prior art attempts to achieve high resolution over an extended ranges have met with only limited success.
Accordingly, it is a general object of the present invention to provide an improved ultrasonic transducer system.
It is a further object of the present invention to provide a simple means of extending the focal depth of a transducer while providing improved focusing characteristics over existing devices.